Compensated radioamplifier for variable load conditions



Feb. 20, 1951 E. D. SMITH ETAL 2,542,293 COMPENSATED RADIO AMPLIFIER FOR VARIABLE LOAD CONDITIONS Filed March 15. 1947 t; D. $5/1/TH W B. REILLY INVENTORS Patented Feb. 2%, i951 i'iED STTES A'iiiNT OFFICE COMPENSATED RADIOAMPLIFIER FQR VARIABLE LOAD CONDITIONS Application March 13, 1947, Serial No. 734,318

9 Claims. 1

This invention relates generally to radio amplifiers, and more particularly to the control and limitation. of the output voltage of radio frequency amplifiers when the latter are in use under variable or unstable load conditions.

An object of this invention is the improvement of the operation and stability of radio frequency amplifiers.

Another object of this invention is to provide for automatic compensation for the deleterious effects of improper or variable load conditions often found in radio transmitter applications and particularly in electronic heating. These deleterious effects as referred to herein comprise excessive anode or plate voltage swing, and excessive screen grid and control grid power dissipation under adverse conditions of load.

A. further object of this invention is the provi ion of means for manually controlling the radio frequency output voltage of a thermionic amplifier tube, in a substantially linear manner, and over a wide range of amplitude.

The term sensitivity as applied to vacuum tube amplifiers, and as herein used means the ratio of output voltage at the load to the input voltage necessary to attain the said output voltage.

The term corrective variation as used herein means any secondary variation, the ultimate effect of which tends to overcome the originating or primary variation.

fill of the foregoing and still further objects d advantages of the invention will become apparent from a study of the following specification, taken in conjunction with the accompanying drawing which illustrates the invention by way of example only and which is a si1nplified schematic diagram of the invention as employed in one of its many forms.

The first vacuum tube V functions as a crystal controll d radio frequency oscillator, supplying an alternating voltage of a predetermined frequency to the control grid of tube V which is rated as a buiier amplifier. Under many cond ions it would be possible to dispense with of these two vacuum tubes, by merely using an oscillator to drive the final amplifier. Similarly, crystal control of frequency is by no means a necessity, as any reasonably stable source of "h frequency energy may be used to replace the oscillator circuit shown.

Vacuum tubes V and V have their anodes at direct current ground potential, while their cathodes and h aters are maintained at a suitable negative potential of 300 volts with respect 2 to ground. This is purely for the sake of convenience and is well known in the prior art.

A GFfi-CT pentode type tube is preferably used at V as a suitable source of high frequency energy. V operates as a standard crystal controlled oscillators, its output voltage being developed across the parallel resonant tank circuit made up of L and C and hence this output voltage or part of it, as desired, appears between the control grid and the cathode of V since C and 0 exhibit negligible reaotance to this alternatin; voltage. Operating bias voltage for V is developed by the flow of control grid current through the grid. leak R It will be noticed that the heaters of V and V are connected to their respective cathodes in order to minimize the potential difference between the heaters and cathodes.

V is pr ferably a 6L6-G beam tetrode type tube. Its screen grid is maintained at radio frequency ground potential by means of the by pass capacitance C As in the case of V the plate of V is connected to ground through the parallel resonant circuit consisting of inductance L and the split-stator capacitance C A reason for returning the plate and screen grid circuits of V and V to ground and connecting the cathode to a source of negative potential is to enable the screen circuit of V and the control grids of the push-pull power output tubes V and V to be brought to a common point on the bleeder resistance constituted by R and R The above mentioned tank circuit comprising L and C is centre-tapped so that V and V may be excited in push-pull. The capacitance C is adjusted to compensate for the output capacitance of V to give balanced output from the tank circuit. This tank circuit may be designed to resonate at the fundamental frequency of the crystal oscillator or at any multiple thereof. In this particular design the frequency of the final output voltage is a fourth harmonic of the crystal frequency, affording greater freuuency stability of output voltage.

frequency voltage at the grids of V and Vt, and yet have a low direct current resistance. They are used to isolate from'the direct current control grid bias circuit the radio frequency signal that is present at the control grids of V and V Resistors R and R are or" such a value that under normal or optimum operating conditions, proper bias is maintained at the control grids of V and V The junction point of R and R is held at radio frequency ground potential by capitance C and is connected to a point on the voltage divider comprising R R and R Resistor B may be called the first resistance and resistors R and R in combination may be called the second resistance.

The plates of V and V are connected to a suitable source of high potential direct current through the parallel resonant tank. circuit consisting of inductance L and the split-stator capacitance Radio frequency energy is isolated from the power supply by the low-pass filter comprising L and C The plate supply voltage, of plus 1500 volts in this particular example, is applied through inductance L to the centre tap on inductance L The external load circuit is link coupled to the tank circuit comprising inductance L and capacitance C by means of the inductance L 7 The screen grid electrodes of the power output tubes are by-passed to ground by capacitors C C and C Operating voltage for the said screen grids is obtained from the plate Voltage supply through the voltage'dr-opping resistance R The values of R R and R are such that under normal or optimum conditions of load irnrpedance the voltages appearing at the control and screen grids of the power output tubes V and V will be as nearly as possible those voltages specified as optimum by the manufacturer of these tubes. In this particular example under normal or optimum conditions of load impedance the screen grid voltage is approximately 300 volts, the voltage at the junction of R and R floating at approximately ground potential.

A suitable supply voltage for the filaments of V and V is developed by a transformer connected to a source of alternating current. These filaments are'held substantially at direct current ground potential by grounding the centre tap on he secondary of the said transformer, and at I radio frequency ground potential by capacitors C C C and C The suppressor grids of V and V are connected directly to ground.

It is evident from the foregoing that the screen grid potential of plus 300 volts is applied to one side of the two series res'istorsR and R which provide the second resistance. The other end of these series resistors is maintained at a constant negative potential of approximately 300 volts. By varying the relative values of resistors R and R the junction point of the two may have any potential from minus 300 volts to plus 300 volts. In the preferred circuit disclosed herein values of all components have been so chosen that under optimum conditions of load impedance and power output, the junction point of resistors R and R floats at approximately ground potential as mentioned above.

The junction point of the screen grids of tubes V and V and the resistors R and R is also connected to the voltage limiting gas discharge tubes V V and V. The sum of the ionization potentials of these series tubes is such that under the above mentioned optimum conditions they remain inoperative. In this particular example three VR-105-30 type tubes have been used so that the total voltage drop from the screen grids of tubes V and V to ground will not exceed approximately 315 volts when the limiter tubes are ionized.

The value of the second resistance comprising resistors R and R is chosen to have a magnitude not less than and preferably much greater than that of the combined cathode-screen grid discharge path resistance of tubes V and V The more R R exceeds in magnitude the aforesaid cathode-screen grid resistance the more nearly will the efiective resistance of the resultant parallel resistance network between cathode and screen grid approach that of the cathode-screen grid discharge path. Hence, any variation in the resistance of the latter has an appreciable effect on the voltage distribution along resistor network formed by R R and R which therefore causes a corresponding shift in the voltage at the junction of R and R This shift, as will be subsequently described, has a direct corrective effect upon the voltages applied to the control grids of the power output tubes.

' The following is'an outline of the operation of the apparatus described by way of example above, when the amount of loadin of the amplifier is varied, and when the desired amount of output power is changed.

Let it be assumed firstly that the amplifier load impedance coupled to inductance L is increased. Due to the mutual inductance existing between inductances L and U, the tank circuit comprising inductance L and split-stator capacitance C will exhibit a higher impedance to the radio frequency current flowing through it. This results in a rise in the amplitude of the plate voltage swing of power output tubes V and V because their plate current swing has remained substantially constant. This increased plate voltage swing causes a decrease in the cathodescreen grid discharge path resistance. The current in the screen grids of the said tubes therefore rises, and consequently the voltage drop acrossresistor R increases. Thus the voltage at the screen grids of tubes V and V drops slightly below its former value. This reduction in screen grid voltage causes the plate current of tubes V and V to decrease slightly, thereby tending to overcome the original rise in plate voltage swing of the power output tubes.

Due to the fact that the voltage divider R R is connected at one end to a source of constant potential with respect to ground (minus 300 volts), the other end of the said voltage divider being connected to the screen grids of tubes V and V, the voltage at the junction of resistors R and R tends to become more negative as the Voltages on the said screen grids decreases.

Because the control grid returns of tubes V and V and the screen grid voltage dropping resistance R of tube V are connected to the junction point of resistors R and R two noticeable efiects are produced as the said junction point becomes more negative with respect to the original potential at the said junction point under the optimum loading conditions. Firstly, the bias voltage on the control grids of tubes V and V becomes more negative, thus reducing the plate current in tubes V and V and thereby reducing the plate voltage swing. Secondly, the voltage at the screen grid of tube V becomes less positive with respect to voltage at the cathode of tube V causing a decrease in its plate current, and thus reducing the excitation applied to the control grids of tubes V and V and thereby further reducing the plate voltage swing at tubes V and V The combined effects in the power output tube of screen grid voltage reduction, control grid bias increase, and decrease in excitation, all tend to substantially decrease the plate current in the said tubes, thereby substantially preventing excessive plate voltage swing and excessive screen grid current.

The above described feedback network is also effective as a protection against overloading of the amplifier. Generally speaking, under overload conditions (load impedance less than the optimum value) the screen grid current in the power output tube decreases, causing a corresponding increase in the voltage at the said screen grid. This in turn causes an increase of plate current in tubes V and V because of the combined efiects of increased screen grid voltage, increased control grid excitation, and decreased control grid bias voltage, thus tending to restore the original plate voltage swing.

In order to limit the amount of rise of the various grid voltages to a safe value, the screens of tubes V and V are connected to the voltage limiting device V V V In this particular case three VR-lO5-30 type gas discharge tubes are used, V V and V remaining inoperative except during conditions of greatly decreased load impedance of the power output plate tank circuit. These voltage limiter tubes operate to prevent the screen grid voltages of tubes V and V from rising above plus 315 volts, since, when the voltage at the screen grids of tubes V and V reaches the discharge potential of the limiter tubes the gas in these tubes becomes ionized and further voltage rise is prevented. 7

It has been found that a very smooth precise control of power output can be obtained by making R variable with a maximum value several times the value of R This means in chest that the potential at the junction of R and R may be varied either positively or negatively with respect to ground over a wide range, affording a smooth manual control of power output. This is particulariy useful for applications that require various conditions of power output such as occur in the use of diathermy, industrial electronic heating and electro-surgery.

This invention also operates as an automatic 1 protection during warm up and standby periods. At these times the minus 300 and heater voltages are applied to the apparatus, but the 1500 volt supply is not yet connected. Under such conditions it is evident that the anodes and screen grids of tubes V and V are disconnected from any direct current path to ground potential, and hence the junction point of resistors R and R assumes a potential of approximately minus 300 volts. screen grid potential of the amplifier tube V with respect to its cathode very greatly reduces the plate current in tube V and consequently negligible excitation is applied to the power output tubes V and V Hence no damage will result to the tubes or components of the amplifier regardless of the duration of the warm up or stand-by periods.

The invention as described in detail above, effectively provides a means of smooth control of power output of an amplifier, and a means of stabilization and protection of the said amplifier when subjected to wide variations of load impedance.

It is thought that the construction and use of This decrease of the 6 the invention will be apparent from the above description of the various parts and their purnose. It will be apparent that changes can be made in the arrangement of parts shown without departing from the fundamental principles upon which the present invention is based. As has already been indicated in the foregoing description, many of the electrical elements shown are for illustrative purposes only and can be replaced by other electrical elements capable of performing similar functions. Thus various changes in. the circuit, and in the shape, size and arrangement of parts may be resorted to without departing from the invention which is defined by the subjoined claims.

What we claim as our invention is:

1. In an amplifier for alternating voltages, an output tube including a cathode, a control grid to which the alternating voltage is applied, a screen grid and an anode; a load impedance connected to the anode, and a source of direct current connected to the load impedance and to the aforesaid cathode; a resistance connected to the aforesaid screen grid, and a source of direct current connected to the resistance and to the aforesaid cathode; a second resistance connected to the screen grid and to the said cathode, the aforesaid control grid being tapped on the said second resistance, said second resistance having a magnitude not less than the magnitude of the cathode screen grid discharge path resistance of the aforesaid vacuum tube so that variations of said discharge path resistance caused by variations of anode load. impedance cause substantial variations of the voltages on said second resistance so as to sheet a variation of the unidirectional bias voltage applied to the control grid of said output tube, said variations being in such a direction as to substantially correct the operating conditions of the tube in. accordance with. the originating variation of load impedance.

2. In an amplifier for alternating voltages, an output tube including a cathode, a control grid to which the alternating voltage is applied, a screen grid and an anode; a load impedance connected to the anode, and a source of direct current connected to the load impedance and to the aforesaid cathode; a second resistance connected to the screen grid and to the said cathode, the aforesaid control grid being tapped on the said second resistance; an amplifier tube adapted to supply the aforesaid alternating voltage to the control grid of the output tube, said amplifier tube having a grid also tapped on the said second resistance; said second resistance having a magnitude not less than the magnitude of the cathode-screen grid dischargepath resistance of the aforesaid vacuum tube so that variations of said discharge path resistance caused by variations of anode load impedance cause substantial variations of the voltages on said second resistance so as to efiect a variation Of the voltages applied to the control grid of said output tube, said variations being in such a direction as to substantially correct the operating conditions of the tube in accordance with the originating variation or load impedance.

3. In an amplifier for alternating voltages, an output tube including a cathode, a control grid to which the alternating voltage is applied, a screen grid and an anode; a load impedance connected to the anode, and a source of direct current connected to the load impedance and to the aforesaid cathode; a resistance connected to the aforesaid screen grid, and a source of direct current connected to the resistance and to the aforesaid cathode; a second resistance connected to the screen grid and to the said cathode; an amplifier tube adapted to supply the aforesaid alternating voltage to the control grid of the output tube, said amplifier tube having a grid tapped on the said second resistance; said second re sistance having a magnitude not less than the magnitude of the cathode-screengrid discharge path resistance of the aforesaid vacuum tube so that variations of said discharge path resistance caused by variations of anode load impedance cause substantial variations of the voltages on said second resistance so as to effect a variation of the voltages applied to the control grid of said output tube, said variations being in such a direction as to substantially correct the operating conditions of the tube in accordance with the originating variation of load impedance.

4. In an amplifier for alternating voltages, an output tube including a cathode, a control grid to which the alternating voltage is applied, a screen grid and an anode; a load impedance connected to the anode, and a source of direct current connected to the load impedance and to the aforesaid cathode; a resistance connected to the aforesaid screen grid, and a source of direct current connected to the resistance and to the aforesaid cathode; a second resistance connected to the screen grid and to the said cathode, the aforesaid control grid being tapped on the second resistance, said second resistance having a magnitude not less than the magnitude of the cathodescreen grid discharge path resistance of the aforesaid vacuum tube so that variations of said discharge path resistance caused by variations of anode load impedance cause variations Of the voltages on said second resistance 50 as to effect a variation of the voltages applied to the control screen grid voltage may vary downwardly from said optimum value but whereby its upward variation is limited to the aforesaid ionization potential of the voltage limiter.

5. In an amplifier for alternating voltages, an output tube including a cathode, a control grid to which the alternating voltage is applied, a

screen grid and an anode; a load impedance connected to the anode, and a source of direct current connected to the load impedance and to the aforesaio cathode; a second resistanceconnected to the screen grid and to the said cathode, the aforesaid control grid being tapped on the said second resistance; an amplifier tube adapted to supply the aforesaid alternating voltage to the control grid of the output tube, said-j amplifier tube having a grid also tapped on the said second resistance; said second resistance having a magnitude not less than the magnitude of the cathode-screen grid discharge path resistance of the aforesaid vacuum tube so that;

variations of said discharge path resistance caused by variations of anode load impedance cause variations of the voltages on said second resistance so as to effect a variation of the voltages applied to the control grid of said output tube, said variations being in such a direction as to substantially correct the operating conditions of the tube in accordance with the originating variation of load impedance; and a voltage limiting gas discharge tube connected be tween the screen grid and the cathode of the output tube the said limiting tube having an ionization potential greater than the screen grid voltage under optimum load conditions, so that screen grid voltage may vary downwardly from said predetermined optimum value but whereby its upward variation is limited to the aforesaid ionization potential of the voltage liniiter.

6. In an amplifier for alternating voltages, an output tube including a cathode, a control grid to which the alternating voltage is applied, a screen grid and an anode; a load impedance connected to the anode, and a source of direct current connected to the load impedance and to the aforesaid cathode;'a resistance connected to the aforesaid screen grid, and a source of direct current connected to the resistance and to the aforesaid cathode; a second resistance connected to the screen grid and to the said cathode; an amplifier tube adapted to supply the aforesaid alternating voltage to the control grid of the output tube, said amplifier tube having a grid tapped on the said second resistance; said second resistance having a magnitude not less than the magnitude of the cathode-screen grid discharge path resistance of the aforesaid vacuum tube so that variations of said discharge path resistance caused by variations of anode load impedance cause variations of the voltages on said second resistance so as to effect a variation of the voltages applied to the control grid of said output tube, said variations being in such a direction as to substantially correct the operating conditions of the tube in accordance With the originating variation of load impedance; and

'a voltage limiting gas-discharge tube connected between the screen grid and the cathode of the output tube, the said limiting tube having an ionization potential greater than the screen grid voltage under optimum load conditions so that screen grid voltage may vary downwardly from said predetermined optimum value but whereby its upward variation is limited to the aforesaid ionization potential of the voltage limiter.

7. In an amplifier for alternating voltages, an output tube including a cathode, a control grid to which the alternating voltage is applied, a screen grid and an anode; a load impedance connected to the anode, and a source of direct current connected to the load impedance and to the aforesaid cathode; a resistance connected to the aforesaid screen grid, and a source of direct current connected to the resistance and to the aforesaid cathode; a second resistance connected to the screen grid and to the said cathode, the aforesaid control grid being tapped on the said second resistance, said second resistance having a magnitude not less than the magnitude of the cathode-screen grid discharge path resistance or the aforesaid vacuum tube so that variations of said discharge path resistance caused by variations of anode load impedance cause substantial variations of the voltages on said second resistance so as to eiiect a variation of the unidirectional bias voltage applied to the control grid of said output tube, said variations being in such a direction as to substantially correct the operating conditions of the tube in accordance with the originating variation of load impedance; and said second resistance being variable so that the magnitude of the unidirectional bias voltage may be manually adjusted to provide a wide range of control over the amplitude of the output voltage across the aforesaid anode load impedance.

8. In an amplifier for alternating voltages, an output tube including a cathode, a control grid to which the alternating voltage is applied, a screen grid and an anode; a load impedance connected to the anode, and a source of direct current connected to the load impedance and to the aforesaid cathode; a second resistance connected to the screen grid and to the said cathode, the aforesaid control grid being tapped on the said second resistance; an amplifier tube adapted to supply the aforesaid alternating voltage to the control grid of the output tube, said amplifier tube having a grid also tapped on the said second resistance; said second resistance having a magnitude not less than the magnitude of the cathode-screen grid discharge path resistance of the aforesaid vacuum tube so that variations of said discharge path resistance caused by variations of anode load impedance cause substantial variations 01" the voltages on said second resistance so as to efiect a variation of the voltages applied to the control grid of said output tube, said variations being in such a direction as to substantially correct the operating conditions of the tube in accordance with the originating variation of load impedance; and said second resistance being variable so that the magnitude oi the unidirectional bias voltage and of the alternating voltage to be amplified may both be manually adjusted to provide a wide range of control over the amplitude of the output voltage across the aforesaid anode load impedance.

9. In an amplifier for alternating voltages, an output tube including a cathode, a control grid to which the load impedance connected to the anode, and a source of direct current connected to the load impedance and to the aforesaid cathode; a resistance connected to the aforesaid screen grid, and a source of direct current connected to the resistance and to the aforesaid cathode; a second resistance connected to the screen grid and to the said cathode; an amplifier tube adapted to supply the aforesaid alternating voltage to the control grid of the output tube, said amplifier tube having a grid taped on the said second resistance; said second resistance having a magnitude not less than the magnitude of the cathode-screen grid discharge path resistance of the aforesaid vacuum tube so that variations of said discharge path resistance caused by variations of anode load impedance cause variations of the voltages on said second resistance so as to efiect a variation of the Voltages applied to the control grid of said output tube, said variations being in such a direction as to substantially correct the operating conditions of the tube in accordance with the originating variation of load impedance; said second resistance also being variable so that the magnitude of the voltage to be amplified may be manually adjusted to provide a Wide range of control over the amplitude of the output voltage across the aforesaid anode load impedance; and a voltage limiting gas discharge tube connected between the screen grid and the cathode of the output tube, the said limiting tube having an ionization potential greater than the screen grid voltage under optimum load conditions, so that screen grid voltage may vary downwardly from said predetermined optimum value but whereby its upward variation is limited to the aforesaid ionization potential of the voltage limiter.

EDWARD DUNSTAN SMITH. WILMO'I' BERTRAM REILLY.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,926,749 Mages Sept. 12, 1933 2,032,199 Braden Feb. 25, 1936 2,211,914 Soller Aug. 20, 1940 2,232,212 Cary Feb. 18, 1941 2,292,136 Lindsay et al. Aug. 4, 1942 2,292,439 Golicke a Aug. 11, 1942 2,367,600 Nelson Jan. 16, 1945 2,420,058 Sweet May 6, 1947 

